The present invention relates to the structure of a shelf to be housed in the frame of a communications apparatus, and more particularly to the structure of a shelf accommodating a plurality of circuit cards (printed circuit boards) mounted with electric circuits.
The frame of a communications apparatus by the prior art typically consists of at least two props, a base holding the bottoms of those props and a connecting frame holding the tops of those props, and a plurality of shelves each accommodating a plurality of printed circuit boards are fixed to the frame in a row along the props. The shelves are open on the front side, from which the printed circuit boards are inserted into the shelves vertically. The shelves have guide members for holding the top and bottom ends of the boards, a back plane having connectors which the printed circuit boards are plugged into, and side plates which are firmly bolted to the guide members and back plane (see "The Bellpac Packaging System" in Western Electric, THE ENGINEER, third issue 1983, pp. 5-15).
The prior art communications apparatus referred to above involves the problem that its frame is deformed into a parallelogram, with its props inclined, by vibration during transportation or by earthquakes. This results in the disadvantage that the side plates of the shelves fixed to the props of the frame are inclined, and the parts fixing the back plane to the guide members and the back plane to the side plates are strained, often inviting the rupture of these parts.
The inclination of the shelves' side plates causes the further disadvantage that the top and bottom ends of the printed circuit boards held by the guide members of the shelves are dislocated in reverse directions to each other, resulting in the distortion of the boards, and consequently the circuit components mounted on the boards may come off or the connecting parts between the boards and the connectors on the back plane may get broken.